Process for the purification of chemical wood pulp



- Patented Oct. 20, 1 931 UNIT-ED 1 STATES PATENT QFFME COMPANY, OF CANTON, NORTEL CAROLINA, A CORPORATION OF OHIO rnocns's non THE ru'nirronrron or CHEMICAL woon rum No Drawing.

This invention relates to processes for the purification or refining of chemical fibrous materials in thecourse of their preparation for the paper,or paper boa-rd, machine.

Sulfate "pulps prepared from coniferous woods possess potentially exceptional strength and toughness but, in their un-.

i ,bleached state, they are brown in color.

Moreover, the development of their strength and toughness normallyis attained .only by a simultaneous development of hardness in the resulting sheet, which quality 'of hardness. tends to give'the sheet a: rigidity and rattle which for many purposes are undesirable.

In, addition, sulfate pulps from coniferous woods contain certain constituents, including coloring matters, which aredifiicultly removable by bleaching, and-easily hydrolyzable 'oellulpses which at least in part are remov-- able by bleaching. The removal of these hydrolyzable; celluloses from the pulps, While contributing to the softness thereof and of the resulting paper, greatly reduces the strength and toughness. As a result, it is possible bythe'usual methods to obtain a product having a high degree of relative whiteness only; at the sacrifice of strength andtoughness, or vice versa.

'It is, of course, well known that treatment of chemical Wood pulps with solutions of caustic alkalies effects the dissolution of a portion of the coloring matter, and of a large Application filed February 21, 1929, Serial No. 341,841.

about 85%,,the normal alpha cellulose content of bleached chemical wood pulp,-to as I have found that chemical WOOCl pulps having an alpha cellulose content of more than approximately 94% are deficient in possess a desirable softness, relative whiteness', and resistance to mechanical treatment. That is to say, while these high alpha celluning trade and for preparing-lacquers and the like, theyma-y not be so desirable for use in paper making. On the other hand, pulps.

having an alpha cellulose content of less than approximately 88% are deficient in softnessand in resistance to mechanical treatment.

An object of the present invention, there= fore, is to provide a process of preparing from unbleached sulfate, or soda, pulp obtained from coniferous wood, a purified pulp E JOHN n. RUE, or AsHEvI LE, ivon'rn CAR LINA, nssre'nonmo THE- CHAMPION FIBRE strength and toughness, although they may.

fill losepulpsare particularly suitable for use in preparing cellulosic solutions for the spincharacterized by having an alpha cellulose L content of from about 88% to about'94=%,.

and especially adapted foruse in paper making by reason of its softness, whiteness, strength, toughness and resistance to mechanical treatment.

, I have found that there are certain conditions as regards the concentration and temperature of the alkaline solution, and certain sequences in the operations of bleaching and alkaline treatment, by the observance of which purified alkaline (i. e., sulfate or soda) pulps having the aforesaid desirable characteristics and "properties may be pro-' duced. As arr-entity, my process comprises the following sequence of operations:

Alkaline '(i. e., sulfate, or soda) pulp 1s subjected to a preliminary treatment with an oxidizing bleaching agent under such conditions,to be set out with greaterparticularity hereinafter,as to represent what I term a drastic bleaching treatment; the resulting partially bleached pulp in aqueous suspension is separated from bleach residues by washing,either Without or preferably with addition of an acid or an acidic reducing agent in such amount as to bring the mixture of partially bleached pulp, bleach residues and suspending liquid to an acidity represented by a pH valve of from 2 to 4;- with the partially bleached and washed pulp, suitably thickened,-e. g., to 15% to pulp substance, there is admixed a relatively concentrated solutionof a strong alkali, e.- g., caustic soda-,in excess, in such amount and concentration as to give the resultlng suspension an alkalinity equivalent to that of a solution of caustic soda of from 5% to 12% strength, and a pulp concentration of from 5% to l0%; the resulting strongly alkaline mixture is brought to and maintained at a temperature of from 7 0 to 212 F., preferably at a temperature of about 125 F., for a relatively short period of time,e. g., 10 minutes to about 6 hours, but preferably for from about one-half hour. to about one hour,with agitation; thereafter the sus-- 'sufficient to give the suspension an acidity represented by a'pI-I valueof from 2 to 4, and

washing,-is given a relatively mild bleaching treatment, preferably at a 3% to consistence, with an oxidizing bleaching agent ;.and the resulting suspension of finally bleached and treated pulp, either without or preferably with addition of an'acid or an acidic reducing agent in an amount sufiicient subjected during the whole sequence of operations.

togive the suspension an acidity represented by a pH value of from 2 to 4', is washed free of bleach residues.

By a drastic bleaching treatment, asthat expression is used in the foregoing general description, I mean such a treatment as efi'ects the major portion ,of the bleaching action to which the alkaline pulp is to be For this purpose, there is used at this stage at least and preferably from 7 5% to 85%,ofthe total oxidizing bleachingagent with which the pulp is to be treated. I prefer to effect this operation with a stock suspensionof from 8% to'12% solids (dry weight),r-using therein a relatively strong solution or'suspension of an oxidizing bleaching agent,-say, "14% of bleaching powder.

, proportions ofloleaching agent would be requiredin the-subsequent stage or stages of bleaching, resulting in the production of byproducts soluble in. alkaline solution.

In such a case, the alpha cellulose content of the fully bleached product would be lowered,.and the pulp would be deficient: in one or more of the properties, strength, softness that I find it desirable to use in the preliminary bleaching stage from one-half to threefourthsor more of the total bleaching agent to be used. Carrying out said preliminary bleachat a stock consistence of between 8% and 12% promotes an economy in time and in heat, and as well improves the quality of the final product.

The adjustment of the pH value of the pulp suspensions to a pH between 2 and 4 by means of an acid or an'acidic reducing agent prior to the various washing operationsis a preferred procedure in that thereby there are caused to be more readily removable by washing those bleach residues and certain other materials which are detrimental to the strengthof the final pulp product, particularly, when allowed to remainin the pulp during the final stages of bleaching! Incom-- plete removal of these residues and materials from the final bleach may result in loss of color and of strength to the final, dried, pulp product. Naturally, where the highest qualities are not demanded, a\satisfactory product can be produced without resorting to one or more of the acidification steps above mentioned.

Among operable acidic reducing agents are included sulfur dioxide, a solution containing stannous chloride inhydrochloric acid,

or the like. The use of acidic reducing agents in removing bleach residues and the like from bleachedpulp is described and claimed generally in the co-pending application Serial No. 289,636, filed June 30, 1928.

Anyother strong alkali may be substituted for the preferred caustic soda in thealkaline treatment stage of the process, provided the 2 quantity and proportion used are chemically equivalent to the caustic soda solution speci-' fied-herein. As stated previously, the caustic soda; solution should have a concentration of between "5% and 12% after the partially bleached pulp has been suspended in it. The reaction between pulp and alkali should take place at a temperature between approximately norgnal room temperature,-say 70 and the boiling point of water; preferably, at about 125 F. Although no appreciable harm is done to the final pulp product if the alkaline treatment is'prolonged for from 4 to 6 hours, the preferred period is one hour, and

may be as short as from 10 to 15.m inutes. This partial bleach prior to the alkaline treatment is essential in the successful carrying out of my process; otherwise, excessive.

In treatment with caustic alkali of high concentration, the saidcaustic alkali has, in addition'to its solvent action, the potential property, of developing in the pulp a condiing can be obtained.

' consists in this, that the use of an excess of strong alkali in the alkaline treatment stage permitsthe re-use of the separated alkaline liquor, after appropriate fortification .with solid caustic or with a highly concentrated solution of caustic, in the treatment of a subsequent charge of pulp.

The above described new process may be practiced upon sulfate or kraft pulp digested in accordance with modern standard practice to effect improvements, as to color, strength, softness, and alpha-cellulose content, over the properties of such pulps when bleached in the manner heretofore known. It has been found, however, that, for the production of a pulp which shall possess the most favorable combination of high whiteness, strength, toughness, softness and alphacellulose content, it is-essential that the unbleached sulfate pulp serving as the starting material of the present invention shall have been prepared under conditions of digestion which distinctly difier from those common in modern practice, especially with respect to' the ratio of chemical to wood, the concentration of the chemical solution broughtinto contact with the wood substance at the beginning of the digestion, and the use or nonuse of black, or spent, digestion liquor. These changes in digestion conditions do not include'alteration in the nature of the chemicals, or in the relative proportion of each chemical used.

The improved digestion procedure above referred to,which procedure forms the subject matter of an application filed August 15, 1929, in the name of George M; Trostel, which subject matter is expressly disclaimed herein,-comprises digesting wood ina cooking liquor characterized in that it contains no spent cooking liquor from a previous digestion, and contains active chemical (c.ygz,

caustic soda and sodium sulfid in about the ratio of 2 to 1) man amount corresponding 1 to from about to about grams per liter, the quantity of active chemical present in said cooking liquor being from about 28 to about 36 pounds for each 100 pounds of wood digested. The weights and proportions of chemicals above setout are expressed as of equivalent sodium carbonate.

The following is a specific example illustrative of a preferred embodiment of the present invention:

Unbleached sulfate pulp, from pine wood digested in the cooking liquor above de-.

scribed, is semi-bleached at approximately 8% consistence at about 100' F., with approximately 14% of bleaching powder. The.

semi-bleached pulp is acidified to a pH value of approximately three, and thoroughly washed. Suflicient caustic 'soda and water j are then added to the pulp to bring the pulp to approximately 5% consistence with a 6.5%

solution of the caustic soda. The inass is agitated for approximately one hour at 125 F., drained, washed, acidified with $0 to a pH value of approximately three, thoroughly washed, bleached with approximately 4% of bleaching powder at 4% consistence, aciditied with S0 to a pH value of approximately three, and thoroughly washed.

The following properties are typical of a pulp prepared according to theabove specific process:

strength of 249; a folding endurance of 82;

a tearing force of 6.5; and, in its preparation, a total bleach requirement of 16%.

By comparison, the following properties are typical of a pulp prepared from the same type of sulfate pulp by the ordinary process of purification. T hedata are expressed inthe same'arbitrary units as were the properties for the pulp prepared by the aid of my process; an alpha cellulose content of 80.8%

a color of 3 parts red, 70 parts green and 63 7 parts blue. The pulpafter 200 ,minutes in the pebble mill was so thoroughly hydrated that-it was practically impossible to prepare sheets of paper from it. The total bleach required was 20.9%. 4 I

No means are at hand for expressing -mathematically the degree of softness of pulps. As judged by the papermaker the pulp prepared by my process'is very much softer than are the pulps, commonly prepared from pulps.

This application is a continuation-in-part of'my application Serial No. 300,961, filed- August 20, 1928.

Lclaim: 1. In the preparation of a purified-chem- ,ical wood pulp using as a starting material an unbleached and unrefined "alkalineprocess wood pulp substantially equivalent to that obtained by digesting wood in a'lcooking liquor characterized in that-it contains active. chemical in an amount corresponding to from equivalent impure chemical about 7 to about 80 grams per liter, the quantity of active chemical present in said cooking liquor being equivalent to from about 28 to about 36 pounds of sodium carbonate for each 100 pounds of Wood digested, and separatingthe resulting pulp substance from spent cooking liquor, the sequence of operations which comprises partially bleaching the I v pulp, washing the. partially bleached pulp,

treating the partially bleached and washed pulpwith a solution of caustic alkali, the pulp washed pulp to a second stage of bleaching treatment and Washing the resulting finally bleached pulp.

2.In vthe preparation of a purified chemical wood pulp using as a starting-material an unbleached'and unrefined alkaline process wood pulp substantially equivalent to that. obtained by digesting wood in a cooking liquor characterized in that it contains active .chemical in an amount corresponding to from about 70 to about 80 grams per liter, the quantity of active chemical present in said cooking liquor being equivalent to form about 28 to about 36 pounds of sodium carbonate for each 100 pounds of wood digested, and separating the resulting pulp substance from spent cooking liquor, the sequence of opera tions which comprises partially bleaching the pulp, washing the partially bleached pulp, treating the partially bleached and Washed pulp with a solution of caustic alkali,

the pulp and solution gether in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5 per cent to about 12 per cent in strength and a pulp concentration of from about 5 per cent to about 10 per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than normal room temperature vbut less than the being brought toboiling point of water, separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treated and washed pulp in aqueous suspension to a sec- 0nd stage of bleaching treatment, adjusting the acidity of the resulting suspending to a pH value of from 2 to 4, and washing the resulting finally bleached pulp.

3. In the preparation of a purified chemical wood pulp using as a starting material an unbleached and unrefined alkaline process wood pulp substantially equivalent'to that obtained by digesting Wood in a cooking liquor characterized in that it contains acwashed pulp with a' solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of fromabout 5 percent to about 12 per cent in strength and a pulp concentration of from about 5 per cent to about 10 per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than normal room temperature but less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washing the latter, adjusting the acidity of the resulting washed pulp to a pH. value of from 2 to 4 and again washing the pulp, subjecting the treated and washed pulp in aqueous suspension to a scc- 0nd stage of bleaching treatment, ad usting the acidity of the resulting suspenslon to a pH value of from 2 to 4, and washing the resulting finaly bleached pulp.

4. In the preparation of a purlfied chemical wood pulp using as a starting material an unbleached and unrefined alkaline process wood pulp substantially equivalent to that obtained by digesting wood in a cooking liquor characterized in that it contains active chemical in an amount corresponding to from about to about 80 grams per liter, the quantity of active chemical present 1n Said cooking liquor being equivalent to from about 28 to about 36 pounds of sodium carbonate for each 100 pounds of wood digested, and separating the resulting pulp substance from spent cooking liquor, the sequence of op-' erations which comprises partially bleaching the pulp, adjusting the acidity of the resulting partially bleached pulp to a pH value of from 2 to 4, washmgthe partially bleached pulp, treating the partially bleached and Washed pulp with a solution of caustic alkali, the pulp and solution bemg brought together in such amounts and concentrations as to give a resulting suspenslon hav ng an alkalinity equivalent to that of a caust1c soda solution of from about 5 per cent to about 12 per cent in strength and a pulp concentratlon of from about 5 per cent to about 10per cent, continuing the treatment for from about ten minutes to about'six hours and at a temperature greater than normal roomtemperature but less than the boiling point of water, separating caustic alkali solution from the sotreated pulp andwashing the latter, adjusting the acidity of the resulting washed pulp to a pH value of from 2 to 4 and again washing the pulp, subjecting the treated and Washed pulp in aqueous suspension to a second stage of bleaching treatment, adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washingthe resulting finally bleached pulp.

5. "In the preparation of a purified'chemical wood pulp using asa starting material an unbleached and unrefined alkaline process wood pulp substantially equivalent to that obtained by digesting Wood in a cooking liquor characterized in that it contains active chemical in an amount corresponding to from about to about grams per liter, the quantity of active chemical present in said cooking liquor being equivalent to from about 28 to about 86 pounds of sodium carbonate for each 100 pounds of wood digested,

and separating the resulting pulp substance from spent cooking liquor, the sequence of operations which comprises partially bleaching the pulp, treating the partially bleached and washed pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts-and concentrations as to give a resulting suspension having an alkalinity equivalent tothat of a caustic soda solution of from about 5 per cent to about 12 per cent in strength and a pulp concentration of from about 5 per cent to about 10 per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than the normal room temperature but less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washingthevlatter, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp.

6; In the preparation of a purified chemical wood pulp using as a starting material an unbleached and unrefined alkaline process wood pulp substantially equivalent to that obtained by digesting wood in a cooking liquor characterized in that it contains active chemical in an amount correspondin to form about 70 to about 80 grams per liter, t e vquantity of active chemical present in said cooking liquor being equivalent to from about 28 to tion being brought together in such amounts 5 per cent to about 12 per cent in strength and a pulp concentration otfrom-about 5 per cent to about 10 per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than normal room temperature but less than the boiling point of water, separating caustic alkali solution from the so-treated'pulp and washing the latter, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp. p v

7. ln the preparation of a purified chemical wood pulp using as a starting material an unbleached and unrefined-alkaline process wood pulp substantially equivalent to that obtained by digesting Wood in a cooking liquor characterized in that it contains active chemical in an amount corresponding to from about 70 to about 80 grams per liter, the quantity of active chemical present in said cooking liquor being equivalent to from about 28 to about '36 pounds of sodium carbonate for each 100 pounds of wood digested, and separating the resulting pulp substance from spent cooking liquor, the sequence of opera-' tlons which comprises partially bleaching the pulp, washing the partially bleached pulp, subjecting the resulting unbleached alkaline wood pulp at a stock consistence of from 8% to 12% to a preliminary bleaching treatment with an oxidizing bleaching agent in an amount equivalent toffrom not less than onehalf to about three-fourths of the total oxidizing bleaching agent required, Washing the partially bleached pulp and suitably thickening the' same, admixing with the suspension a solution of caustic alkali of such con-' centration and in such proportion'with respect to the suspension that the resulting mixture has an alkalinity equivalent to a solution of caustic soda of from 5% to 12% strength, and a pulp concentration of from 5% to 10%, maintaining the mixture at a temperature above 70 and below 212 F. for a 8. 'In the preparation of a purified chemicalvwood pulp using as a starting material an unbleached and unrefinedalkaline process wood pulp substantially equivalent to that obtained by digesting wood in a cooking liquor characterized in that it contains active chemical in an amount corresponding to from about 7 0 to about 80 grams per liter, the quantity of active chemical present in said cooking liquor being equivalent to from about 28 to about 36. pounds of sodium carbonate for each 100 pounds of wood digested,

and separating the resulting pulpsubstancel from spent cooking liquor, the sequence of operations which comprises partially bleaching the pulp, washing the partially bleached pulp, subjecting the resulting unbleached sulfate pulp at a stock consistence of from 8% to 12% to a preliminary bleaching treata strength, and a pulp concentration'of from 5% to 10%, maintaining the mixture at a temperature above 70 and below 212 F. for

a period of from 10 minutes to 6 hours, sep

arating caustic alkali solution from the treated'pulp in said mixture andwashing the pulp, subjecting the treated and washed pulp in an aqueous suspension having a conthe partially b sistence of from 3% to. 4% to a secondary bleaching treatment with an oxidizing bleaching agent in an amount equivalent to from not more than one-half to about onefourth of the total oxidizing bleaching agent required, adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting purified pulp.

9. In the preparation of a purified chemical wood pulp using as a starting material an unbleached and unrefined alkaline process wood pulp substantially equivalent to that obtained by digesting wood in a cooking liquor characterized in that it contains active chemical in an amount corresponding to from about 7 0 to 80 grams per liter, the quantity of active chemical present in said cooking liquor being equivalent to from about 28 to about 36 pounds of sodium carbonate for each 100 pounds of wood digested, and separating the resulting pulp substance from spent cooking liquor, the sequence of operations which com prises partiall bleaching the pulp, washing leached pulp, subjecting the resulting unbleached sulfate pulp at a suspension consistence of about 8% to a preliminary bleaching treatment with approximately 14% of bleaching powder based upon the weight of the dry pulp substance at about 100 F., acidifying the pulp suspension to a pH value of about-3, washing the so-acidified pulp and suitably thickening the same, admixing therewith caustic soda solution in such amount and concentration as to bring the resulting mixture to aconsistence of approximately 5% of dry pulp substance with an alkalinity of the resulting solution equivalent to 6.5% of caustic soda, maintaining the mixture at about 125 F. for about one hour with agitation, draining caustic soda solution from the pulp in said mixture and washing the pulp, acidifying the pulp to a pH value of about 3 and again washing the pulp,

ing the partially bleached pulp,,treating the 7 partially bleached and washed pulp with a solution of caustic alkali, the pulp and solution=being brought together in such amounts andconcentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5 percent to about 12 per cent in strength and a pulp concentration of from about 5 per treatment-for from about ten. minutes to about six hpurs, separating caustic alkali solution from the so-t'reated pulp and washcent to"about 10 per cent, continuing the 1 ing the latter, subjecting the treated and f washed pulp to a second stage of bleaching treatment, and washing the resulting finally bleached pulp.

11. In purifying unbleached chemical wood pulp, the sequence of operations which comprises partially bleaching the pulp, washing the partially bleached pulp, treating the partially bleached and washed pulp with a solution of caustic alkali,'the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension havlng anealkahnity equivalent to.

that of a caustic soda solution of from about 5 per cent to about 12 per cent in strength and a pulp concentration of from about 5 'per cent to about 10 per cent, continuing the treatment for from aboutten minutes to about sixhours and at a temperature greater than normal room temperature but, less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treated and washed pulp in aqueous suspension to, a

second stage of bleaching treatmentyadjusting the acidity of the resulting suspension to a pH value of'from 2 to 4, and washing the resulting finally bleached pulp.

12. In purifying unbleached chemical wood pulp, the sequence of operations which comprises partially bleaching the pulp, washing the partially bleached pulp, treating the partially bleached and washed pulp with a solution of causticalkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about. x per cent to about 12 per cent in strengthv and a pulp concentration of from about 5 per cent to about per cent, continuing the treatment for from about ten minutes to about six hours at a temperature greater than norsuspension to a pH value of from 2 to- 4, and' washing the resulting finally bleached pulp.-

13. In purifying unbleached chemical wood pulp, the sequence of operations which comprises partially bleaching the pulp, adjusting the acidity of the resulting partially bleached pulp to a pH value offrom 2 to 4, washing. the partially bleached pulp, treating the partially bleached and washed pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give'a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5 per cent to about 12 per cent in strength and a pulp concentration of from about 5 per cent to about '10 per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than normal room temperature but less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washingthe latter, adjusting the acidity of the resulting washed pulp to a pH value of from 2 to 4 and agaln washing the pulp, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, 5 adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp.

14. In purifying unbleached ulp prepared from coniferous wood by an a aline process, the sequence of operations which comprises partially bleaching the pulp, washingthe partially bleached pulp, treating the artially bleached and washed pulp with a so ution of caustic alkali, the pulp and solution being brought together in such amounts and con-.

having an alkalinity equivalent to that of a caustic soda solution of from about 5 per cent to about 12 per cent in strength and a pul concentration of from about 5 per cent to a out 10'per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than the normal room temperature but less than the boiling point of'water. separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp.

15 In purifying unbleached pulp prepared from coniferous wood by the sulfate process, the sequence of operations which comprises partially bleaching the pulp, washing the partially bleached pulp, treating the partially bleached and washed pulp with a solutionjof caustic alkali,,the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a causticisoda solution of from about 5 percent to about 12 per cent in strength and a pulp concentration of from about 5 per cent to about 10 per cent, continuing the treatment for from about ten minutes to about six hours and at a temperature greater than normal room temperature but less than the boiling point of'water, separating caustic alkali solution from the so-treated pulp and washing the latter. subjecting the treated and centrations as to give a resulting suspension washed pulp in aqueous suspension to a sec- 0nd sta e of bleaching treatment, adjusting the aci ity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp. e In testimony whereof, I alfix my signature.

,JOHN D, RUE. 

